I had the same question few weeks back.
It is a secreat, as I got to know by reading some forum posts and FAQ. HubPages team keep it as a secreat, since if it available to everyone, people do some rubbish things to improve their HubScores. As I understand writing original hubs, actively involve in Forums, reading and commenting on others hubs, average hubscore of hubs and incoming traffic from outside hubpages will improve your score.
My hubscore went down to 60s from 80s after flagging 10 hubs of mine as wrong catergorized. It still remain in 60s for last 2 weeks. Their is no effect to my traffic, therefore now I do not worry about my score.